How a rebar grid mat is counted and ordered
Understand how a slab rebar grid is counted, why an edge bar is added each way, and how to turn linear feet into sticks with a splice allowance.
What a grid mat is
A slab is usually reinforced with a single flat mat of bars crossing at right angles, tied where they meet. One set of bars runs the length of the slab and the other runs the width, and together they resist the cracking that plain concrete cannot. This calculator models exactly that layout: a uniform grid with one bar spacing used in both directions. It does not model beams, footings or multiple stacked mats, which need their own detailing.
Why the count adds an edge bar
It is tempting to divide the span by the spacing and stop, but that only counts the openings, not the bars framing them. A 10 foot width divided by a 1 foot spacing gives 10 openings, yet you need 11 bars to close both edges. The tool floors the division and adds one, so every reported count already includes that closing bar on the far edge. This is the same reason a fence with 10 sections needs 11 posts.
From linear feet to a shopping list
The linear feet figure is the sum of every bar length in the mat, but rebar is sold in fixed sticks, often 20 feet long. Divide the linear feet by your stick length and round up to get a raw stick count. Then add a lap allowance wherever bars are joined end to end, because a splice overlaps two bars by roughly 40 diameters, and add a few percent for cutting waste. For a 430 foot mat that is about 22 twenty-foot sticks plus your splice and waste margin.
Choosing a spacing and bar size
Tighter spacing and larger bars both add steel and cost, so the right choice balances the load against the budget. Light residential flatwork like a patio may use #3 or #4 bar on a 16 to 18 inch grid, while a driveway or garage pad often steps up to #4 or #5 on a 12 inch grid. These are common ranges, not a substitute for engineering: slab thickness, soil support and expected loads all change the answer, so confirm the spec with a professional or your local code before ordering.